The present invention relates to heat exchangers, and more particularly to a heat exchanger adapted to be coupled between the exhaust outlet of a room heating unit and a chimney for extracting heat from the exhaust gases flowing therethrough and returning the same to the room.
It is well known that room heating units, such as stoves, furnaces, and the like, lose a large proportion of their generated heat through their chimneys. Numerous heat exchanger devices have been invented in the past for extracting heat from the exhaust gases flowing out of a room heating unit so that the same can be returned to the room thereby resulting in substantial energy savings.
U.S. Pat. No. 2,468,909 shows in FIG. 2 a heat exchanger 10 mounted on the flue gas pipe 11 of an oil burning furnace 12. A fan 20 blows air around the flue pipe 11 and into a conveying pipe 29. U.S. Pat. No. 2,962,218 shows in FIG. 1 a heat exchanger 32 which encircles the flue sections 27 and 28 of a gas furnace 10. A blower 15 draws fresh air through a pipe 46 into the heat exchanger 32 and out of the heat exchanger through a pipe 50.
U.S. Pat. No. 3,944,136 shows in FIGS. 1, 2 and 3 a heat exchanger 32 adapted to extract heat from the stack flue sections 12 and 14 of a conventional gas furnace. The flue gases flow through a plurality of pipes 36 in the heat exchanger 32. Ambient air from the furnace room is drawn by a fan 44 around the pipes 36. U.S. Pat. No. 4,044,950 shows in FIG. 1 a heat exchanger 37 connected intermediate flue pipe sections 21. The heat exchanger 37 has baffles 43 defining a helical air passage about a centrally disposed flue extension pipe 39. Air is blown through the heat exchanger by a blower 57.
U.S. Pat. No. 3,813,039 shows in FIGS. 1 and 2 a heat exchanger which draws hot exhaust gases from a furnace exhaust pipe 10 through a plurality of rows of vertically extending tubes 24, 31 and 35. Adjacent ends of the tubes are coupled to form a serpentine path and exhaust gases from the heat exchanger are re-introduced into the output section 12 of the furnace exhaust pipe. A blower 45 forces air past the tubes 24, 31 and 35 in a serpentine path as indicated in FIG. 2.
U.S. Pat. No. 4,103,735 shows in FIG. 1 a heat exchanger 1 adapted to be utilized in conjunction with the exhaust duct 2 of a furnace F. The furnace exhaust is channelled from the exhaust duct 2 through a plurality of horizontally extending vertically spaced pairs of tubes 4 and out of the exchanger through a chimney duct 10. As shown in FIG. 2, a fan 9 draws air into a housing 22 surrounding the tubes 4 and circulates the air lengthwise about the tubes. The heated air exits through an exhaust duct 7 in the housing 22. The heat exchanger 1 has replaceable end caps 8 and 8' which can be removed to permit cleaning of the heat exchanger.
The following U.S. Pat. Nos. relate to heat exchangers and other apparatus in this field, however, none appears to be any more pertinent to the present invention than the patents discussed above: 931,565; 1,565,032; 1,953,302; 2,190,410; 2,244,055; 2,252,784; 2,267,905; 2,290,255; 2,348,569; 2,362,940; 2,378,181; 2,508,131; 2,527,937; 2,555,842; 2,674,240; 2,711,683; 2,715,018; 2,738,785; 2,764,391; 2,893,374; 3,106,241; and 3,124,197.
While the six patented heat exchangers summarized above improve the energy efficiency of a room heating unit to some extent, each utilizes a fan or blower which itself requires energy to operate. They have not been designed to circulate air by convection. Furthermore, in those patented heat exchangers in which the hot exhaust gases are conveyed through a plurality of tubes, the same air is generally circulated past the entire set of tubes. Thus, some of the heated tubes come into contact with relatively cool room air while others come into contact with air which has been preheated by other tubes past which the air has already circulated. Greater heat exchange efficiency can be achieved if air at or near room temperature is circulated past a larger proportion of the tubes.